1. Field of the Invention
The present invention relates to a method of manufacturing a solid state imaging device, and relates more particularly to a method of manufacturing a phase grating image sensor.
2. Description of the Related Art
In the recent trend toward a compact solid state imaging device, there has arisen a problem of decreased photosensitivity of the solid state imaging device because the area of a light receiving portion is reduced as the device becomes compact. As a countermeasure against this problem, a solid state imaging device comprising a micro lens for collecting light on the light receiving portion has already been realized. The technique for forming a micro lens is indispensable in the production of a solid state imaging device at present.
FIG. 1 is a sectional view showing an exemplified configuration of the conventional solid state imaging device. A conventional solid state imaging device will now be described. In FIG. 1, reference numeral 100 denotes a semiconductor substrate, reference numeral 102 denotes photodiodes formed on the surface of the semiconductor substrate 100 so as to convert incident light into a charge, reference numeral 104 denotes a first flattening layer for flattening the surface of the semiconductor substrate 100, a reference numeral 106 denotes a color filter layer formed on the flattening layer 104, reference numeral 108 denotes a second flattening layer for flattening the level difference on the color filter layer 106, and reference numeral 110 denotes micro lenses formed on the second flattening layer 108 so as to collect light on the corresponding photodiodes 102.
The first flattening layer 104 is formed by coating the semiconductor substrate 100 with a transparent film material in a desired thickness. The color filter layer 106 is formed by, for example, photolithography methods so as to correspond to the respective photodiodes 102. The second flattening layer 108 is also formed by coating the color filter layer 106 with a transparent film material in a desired thickness.
Each of the micro lenses 110 is made of a phenol resin or the like, and is formed above the corresponding photodiode 102 in the shape of a hemisphere. The micro lens 110 has an appropriate height so that light entering the surface thereof can be efficiently collected on the corresponding photodiode 102.
The hemispherical shape of the micro lens 110 is formed through the following procedures: First, a lens resin is coated on the second flattening layer 108. Then, the lens resin is subjected to an exposing treatment using a lens mask and further to a developing treatment, thereby patterning the lens resin so as to be located in the position above each of the photodiodes 102. Then, the patterned photo resist is heated so as to be melted, thereby forming the hemispherical shape of the micro lens 110 by making use of the surface tension.
However, the aforementioned solid state imaging devices and the production methods for the devices have the following problems. In the solid state imaging device of FIG. 1, for the purpose of further improvement of the photosensitivity, it is desired to make a space between the adjacent micro lenses 110 as small as possible so as to enlarge the light receiving area of each micro lens 110.
In the conventional production method, however, when the micro lenses 110 are formed by heating the patterned lens resin (hereinafter referred to as the lens patterns), the heating temperature is set at which the lens patterns are completely melted. Therefore, the melted lens patterns are forced out of the bottoms of the intended patterns.
Furthermore, in the case where the space between the adjacent lens patterns is too small, the adjacent lens patterns which have been forced out of the intended patterns due to the thermal dissolution come in contact with each other, and eventually the lens patterns flow out. This spoils the shape of each micro lens 110, resulting in a smaller surface area of the hemisphere portion as well as a smaller height. The photosensitivity of the device could degrade as the light quantity collected on each photodiode 102 decreases.
Moreover, since the lens patterns are completely melted in the conventional methods, it is impossible, due to the surface tension, to make the height of the micro lens 110 larger than a half of the bottom width of the micro lens 110 in the alignment direction of the light receiving portions. Accordingly, when a distance between the micro lens 110 and the photodiode 102 is small, it is probable that the micro lens 110 cannot be formed into an optimal shape for collecting the light.
In view of the drawbacks mentioned with respect to the prior art process, there is a continuing need to develop new and improved processes that overcome the disadvantages associated with prior art processes. The advantages of this invention are that it solves the problems mentioned above.
It is therefore an object of the invention to provide a method of manufacturing a phase grating image sensor by merely using conventional photolithography methods.
It is another object of this invention to provide a method of manufacturing a phase grating image sensor to form a phase grating lens instead of a micro-lens.
It is a further object of this invention to provide a method of manufacturing a phase grating image sensor that can avoid the problem of micro-lens shape control resulting from the increasing integration and the conventional processes.
To achieve these objects, and in accordance with the purpose of the invention, the invention uses a method of manufacturing a phase grating image sensor. The method comprises the following steps: providing a substrate having a plurality of photosensitive devices thereon, a first flattening layer on the photosensitive devices, a color filter layer on the first flattening layer and a second flattening layer on the color filter layer, wherein the first flattening layer and the second flattening layer are transparent for the incident beam; forming a photoresist layer over the second flattening layer; transferring a plurality of phase grating patterns into the photoresist layer, wherein each the phase grating patterns comprising a plurality of concentric circles and corresponding to each respective underlying photosensitive device; anisotropically etching the second flattening layer to a predetermined depth to form a plurality of phase grating lenses; and removing the photoresist layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.